Preparation of benzene carboxylic acids



United States No Drawing. Filed Sept. 30, 1957, Ser. No. 686,894 3 Claims. (Cl. 260-523) The present invention relates to a process for the preparation of aromatic carboxylic acids and more specifically the present invention relates to an improved method of oxidizing in the presence of molecular oxygen and in the liquid phase benzene compounds containing at least one partially halogenated alkyl radical to their corresponding carboxyl benzene compounds.

In the past numerous methods have been attempted to bring about the airvoxidation of aromatic compounds to the corresponding aromatic acids especially xylenes to phthalic acids. These oxidation reactions have generally been carried out in the presence of a catalyst such as cobalt toluate, cobalt naphthenate, etc. The main difficulty with these reactions, however, has been the fact that the ultimate yield of polycarboxylic acids has been low, e.g. about 20 percent in a one-stage process with the preponderance of the product being the monocarboxylic acid. In order to go to the dior polycarboxylic acid it is usually necessary to esterify the acid, halogenate the alkyl groups or alternatively employ a chemical oxidizing agent such as nitric acid. Recently issued Australian patent specification No. 16,742/56 describes a method of oxidizing aromatic compounds to aromatic acids. This patent provides a good one-stage method for the liquid phase air oxidation of alkyl substituents to polycarboxylic aromatic acids by the use of a metal, e.'g. cobalt and bromine oxidation catalyst.

An article appearing in Industrial and Engineering Chemistry, vol. 41, No. 11, page 2615, entitled Oxidation of Aromatic Compounds, is also of interest on this subject and discloses a method for etlecting the liquid phase oxidation of alkyl aromatics in the presence of hydrogen bromide. The oxidation process described there in is conducted in the absence of a solvent and the acid product is substantially contaminated with various impurities such as brominated reaction products and various phenolics. The patent to Rust et al. US. Patent No. 2,415,800 also discloses the oxidation of alkyl aromatics while in the presence of hydrogen bromide. In this patent the oxidation reaction is generally carried out in the vapor phase and in the absence of a solvent. The hydrogen bromide apparently acts as an inhibitor to retard the decomposition of the hydrocarbon feed stock in order t obtain oxidation products having the same number of carbon atoms as the feed stock.

In accordance with the present invention, I have found an alkyl substituted benzene compound is capable of being oxidized in the liquid phase at temperatures of about 50 to 350 C. with molecular oxygen and that high yields of its corresponding carboxylic acid can be obtained provided that the benzene compound comprising the feed stock have at least one partially halogenated alkyl substituent therein and further provided that the oxidation reaction be carried out while in the presence of catalytic amounts of hydrogen bromide and a solvent selected from the group consisting of acetic acid and benzene or mixtures thereof. In many instances the conversion of the feed stock can be 100 percent or substantially quantitative.

By a benzene compound having at least one partially halogenated alkyl radical therein, I intend to mean any alkyl substituted benzene hydrocarbon in which a hydrogen trom the alkyl side chain has been substituted by a atent ICC halo radical such as chlorine or bromine, and preferably the latter. The benzene compound must have at least one halo alkyl radical attached to the benzene nucleus and it can contain a plurality thereof if desired. For instance, the benzene nucleus can be substituted with as many as six halogenated alkyl side chains. The alkyl radical to be oxidized can contain from one to four carbon atoms and preferably, it will be methyl. Moreover, the benzene ring can contain other substituents such as higher hydrocarbon side chains, bromine, etc. The following are representative halogenated alkyl substituted benzene compounds which are capable of being oxidized in accordance with the present invention: beuzyl bromide, bromoxylene, dibromoxylene, tribromo trimethyl benzene, tetrabromodurene and bromoethyl benzene, bromodiethyl benzene, dibromodiethyl benzene, bromobutyl benzene, dibromodibutyl benzene, etc. The partially halogenated alkyl substituted benzene compounds can be obtained in any conventional manner. For example, the desired alkyl benzene compound can be brominated in either the liquid or vapor phase by direct contact with molecular bromine while in the presence of ultraviolet light and the extent of bromination can be controlled by observing the changes in molecular weight. It is preferred that the alkyl side chain have only one halo group attached thereto. The halogenated products obtained by this or similar procedures can be separated and purified as by distillation or the crude reaction mixture can be sent directly to the oxidation reaction zone.

In order to obtain the desired yields of carboxylated acids in a high degree of purity, it is necessary that the reaction be conducted in a medium which affects the solubilization of the reactants. More particularly, the oxidation reaction is carried out in the presence of a solvent selected from the group consisting of acetic acid and benzene, substituted derivatives thereof such as trichloroacetic acid or any mixture of these materials. The reaction medium can be either anhydrous or hydrated. No particular advantageous or deleterious results are noticeable, however, by the deletion or inclusion of water in the system. The amount of solvent can vary over a wide range and is usually provided in amounts sufficient to solubilize a substantial amount of the feed stock. The amount of solvent will primarily depend upon the choice of solvents and the solubility of the'particular feed stock therein. It is not necessary that the solvent be provided in amounts sufiicient to solubilize the entire charge of feed stock but enough must be present to render a suflicient amount of the feed stock in solution in order to effect the oxidation, with additional amounts of the feed stock being solubilized as the reaction products are formed. Generally, the solvent will be provided in amounts ranging from about 25 to 2000 percent by weight of the feed stock and preferably about to 500 percent by weight.

As mentioned previously, the oxidation reaction of the present invention must be carried out in the presence of hydrogen bromide. By this I do not intend to mean that hydrogen bromide as such must always be introduced into the system since in some instances, as for example, when oxidizing a brominated feed stock extraneous amounts of hydrogen bromide or hydrogen bromide-producing substances need not be provided and the reaction will proceed to substantial completion. However, if the feed stock has been halogenated as with chlorine, I find it desirable to introduce catalytic amounts of hydrogen bromide along with the feed stock. If desired, hydrogen bromide per se need not be provided but it can be formed in-situ as by the introduction of substances such as free bromine, hypobromous acid, etc., which under the reaction conditions will yield the necessary amount of hydrogen bromide. The hydrogen bromide will be provided in amounts sulficient to catalyze the reaction and can vary over a wide range with no particular limits capable of being ascertained and is effective in the absence of catalytic metals such as cobalt. Generally, the catalyst will be provided in amounts ranging from as low as 0.0001 mol percent up to as high as about 25.0 mol percent based on the feed stock. Preferably, however, the hydrogen bromide should be available in amounts ranging from about 0.1 to 5.0 mol percent.

The conditions under which the oxidation reaction is conducted are, for example, temperatures of about 50 to 350 C. and preferably about 100 to 250 C. with a pressure on the system sufficient to maintain the liquid phase at the operating temperature, usually about to 3000 p.s.i.g. and preferably about 100 to 500 p.s.i.g. sufficing. Oxygen or oxygen-enriched gas such as air can be introduced into the reaction zone in order to efiect the oxidation. The conversion in such a system will usually be complete after contact periods ranging from one minute up to about hours.

The hydrogen halide produced in the halogenation and the oxidation of the benzene compounds can be converted to elemental halogen for reuse in the halogenation stage by any of the conventional procedures such as by passing air or other similar oxygen-containing gas together with the hydrogen halide over a catalyst. This catalyst may be of the deacon type, i.e. pumice impregnated with copper chloride or copper bromide.

By way of illustrating the present invention there are presented below several specific examples of my oxidation procedure and the results obtained thereby.

The examples show only the oxidation step and not the halogenation of the aromatic hydrocarbon nor the regeneration of the elemental halogen from the halogen halide since these are well-known conventional procedures.

Example I 3.5 grams of benzyl bromide and 18 grams of glacial acetic acid were charged into a 120 ml. glass tube sealed at both ends and mounted inside a 300 ml. shaker bomb. The bomb was closed and oxygen-enriched gas (60% oxygen-40% nitrogen) was introduced through a small hole in the top of the glass tube until a pressure of about 300 p.s.i.g. was obtained. Heat was applied and the bomb agitated until a temperature of about 190 C. was obtained. This temperature was obtained in about 40 minutes and was maintained for about 3 hours. After this heating period the bomb was cooled and the glass tube removed. Cold water was added to the solution and the benzoic acid crystallized therefrom. The crystals were removed by filtration, dried and weighed. A yield of 2.2 grams (theory 2.5 grams) of substantially pure (M.P. 122 C.) benzoic acid was obtained. No allowance was made in this or the subsequent examples for mechanical losses.

Example 11 6.4 grams of alpha, alpha dibromo-p-xylene and 18 grams of glacial acetic acid were charged into the glass tube of Example I. The oxidation conditions were substantially the same as Example I. After cooling the insoluble terephthalic acid was removed by filtration and washed with water and alcohol. The brilliant white crystals were dried at 125 C. and weighed. A recovery of 3 grams (theory 4 grams) was obtained. The analysis 4 of the terephthalic acid checked well with the theoretical values, the difference being within experimental error.

Found Theory Percent Carbon 57. 3 57.7 Percent Hydrogen 3. 7 3. 6 Saponification No 677.0 675.0

Example III values with the differences being within experimental error:

Found Theory Percent Carbon 57.4 57.7 Percent Hydrogen 3. 6 3. 6 Saponification No 673.0 675. 0

Example IV 2.0 grams of alpha, alpha dichloro-p-xylene, 18 grams of glacial acetic acid and 0.09 ml. of aqueous hydrogen bromide (48%) were charged into the glass tube of Example I. The oxidation conditions and recovery procedures were the same as that employed in Example I. A yield of 21 percent high purity terephthalic acid was obtained.

It is claimed:

1. A method for the preparation of benzene carboxylic acid which comprises oxidizing in the presence of molecular oxygen a liquid phase reaction mixture consisting essentially of benzene having at least one partially brominated alkyl radical, said alkyl radical containing from 1 to 4 carbon atoms and an acetic acid solvent at a temperature of about to 350 C.

2. A method for the preparation of a phthalic acid isomer which comprises oxidizing in the presence of molecular oxygen a liquid phase reaction mixture consisting essentially of the corresponding alpha, alpha dibromoxylene isomer and an acetic acid solvent at a temperature of about 100 to 350 C.

3. A method for the preparation of benzene carboxylic acid which comprises oxidizing in the presence of molecular oxygen a liquid phase reaction mixture consisting essentially of an alpha bromoxylene and an acetic acid solvent at a temperature of about 100 to 350 C.

References Cited in the file of this patent UNITED STATES PATENTS 2,245,528 Loder June 10, 1941 2,444,924 Farkas et al. July 13, 1948 2,480,971 Rust et al. Sept. 6, 1949 2,833,816 Satfer et al. May 6, 1958 FOREIGN PATENTS 741,380 Great Britain Nov. 30, 1955 

1. A METHOD FOR THE PREPARATION OF BENZENE CARBOXYLIC ACID WHICH COMPRISES OXIDIZING IN THE PRESENCE OF MOLECULAR OXYGEN A LIQUID PHASE REACTION MIXTURE CONSISTING ESSENTIALLY OF BENZENE HAVING AT LEAST ONE PARTIALLY BROMINATED ALKYL RADICAL, SAID ALKYL RADICAL CONTAINING FROM 1 TO 4 CARBON ATOMS AND AN ACETIC ACID SOLVENT AT A TEMPERATURE OF ABOUT 100 TO 350*C. 